The long term objective of this investigation is to determine whether interferon plays a role in limiting the spread of and preventing cancer in man. We propose to investigate two possible ways in which it might act. First, interferon might inhibit the multiplication of RNA tumor viruses if such viruses cause cancer in man. We therefore propose to investigate the mechanisms by which interferon inhibits the multiplication of RNA tumor viruses. Second, interferon has recently been shown to possess cell-multiplication inhibitory (CMI) activity, and there is evidence that tumor cells are more sensitive to interferon than normal cells. We therefor propose to investigate the mechanisms by which interferon inhibits the multiplication of normal and tumor cells, and to determine whether the CMI activity of interferon can be directed specifically against tumor rather than normal cells.